It is possible to convert renewable energy such as wind, wave, tidal energy or water current flows into electrical energy by using a turbine to drive the rotor of an ac generator, either directly or by means of a gearbox. The ac frequency that is developed at the stator terminals of the generator (the “stator voltage”) is directly proportional to the speed of rotation of the rotor. The voltage at the stator terminals also varies as a function of speed and, depending on the particular type of generator, on the flux level. For optimum energy capture, the speed of rotation of the output shaft of the renewable-energy turbine will vary according to the speed of the wind or water current flows driving the blades of the turbine assembly. Matching of the variable voltage and frequency of the generator to the nominally constant voltage and frequency of the power network can be achieved by using a power converter.
The power converter may have any suitable topology (e.g. a two- or three-level pulse width modulated inverter) and is typically connected to the generator by a suitable circuit. For example, the circuit may include one or more conductors or cables for each phase of the generator.
A typical ac synchronous generator includes a field system mounted on the rotor surrounded by a stator winding mounted on the stator. The stator winding may be formed from one or more separate windings, each having n phases. Three phases (i.e. n=3) would be typical, but other phase numbers are possible in some cases. Each winding includes a plurality of coils that are located in winding slots formed in a surface of the stator assembly. The rotor provides a rotating magnetic field generated by conventional windings with slip rings or brushless excitation power supply. The turbine drives the rotor to rotate and ac power is provided by the stator winding.
Such an arrangement might experience a number of different faults during its operation. For example, a short circuit could occur between two or more of the conductors that form the n-phase circuit between the generator and the power converter, or within the power converter itself. In the event of a short circuit or fault then the generator will develop a fault current that can cause unacceptable damage if it is not properly controlled.
In the case of a generator where the rotating field system is provided by a field winding then the rotor flux can very quickly be set to zero by the power converter or external circuitry. This will also set the stator voltage to zero and remove the fault current.
For renewable energy applications then permanent magnet generators offer considerable benefits such as reduced losses, improved efficiency, and the ability to operate at very low rotational speeds so that the gear box between the turbine assembly and the rotor can either be eliminated completely or reduced in complexity. However, if the rotating field system uses permanent magnets then the rotor flux remains substantially constant at all times. This means that if a fault current develops then it cannot be reduced by the power converter or external circuitry. The only way to bring the fault current to zero is to bring the rotational speed of the turbine assembly to zero by controlling the turbine blades. It will be readily appreciated that for large wind turbines this cannot be done quickly and it might take several seconds to bring the generator rotor to a complete stop. A fault current can therefore inflict significant damage on the generator, e.g. by de-magnetizing the permanent magnets which would make the generator incapable of producing electrical energy.
A particular concern is where a fault current is developed as a result of a short circuit between less than n conductors of the n-phase circuit between the generator and the power converter. For example, in the case of a typical circuit where the stator winding has three phases then the effect on the generator can actually be more severe if the short circuit is between just two of the conductors than if it is between all three of the conductors. A short circuit between less than n conductors can result in severe overheating of the permanent magnets and high levels of vibration that can damage the turbine assembly.
One way of eliminating the risk of de-magnetizing the permanent magnets is to raise the electrical impedance of the generator. This can reduce the fault current to a magnitude that will not de-magnetize the permanent magnets. Fuses have also been included as part of the n-phase circuit but they give little practical protection since the raised electrical impedance of the generator typically results in a fault current that is too low to trip and cause the fuses to blow.
Another option is to use series contactors (e.g. a relay device with a contact for each phase), or similar switching devices, in the n-phase circuit between the generator and the power converter. The most recent designs of permanent magnet generators for renewable energy applications, and in particular those that operate without gearboxes, have a very low rotational speed and the ac frequency that is developed at the stator terminals of the generator is also very low. The contactors must therefore have the ability to interrupt the fault current at these low frequencies, which requires the use of very expensive DC rated contactors.